A common problem in electrocardiography monitoring or acquisition is the contamination of the ECG signal with power line noise, wideband noise, or baseline wander. These distortions render the electrocardiogram difficult to read.
Power line noise is typically caused by environmental interference with the ECG device at about 50 Hz or 60 Hz. A common solution to correct the ECG for power line noise has been to pass the signal through a bandstop filter which rejects signal components at the power line frequency. However this filter has the disadvantage that it can cause ringing in the vicinity of the QRS complex of the ECG signal. Another solution is to employ an adaptive noise canceler to reduce or eliminate power line interference. This solution is less likely to cause objectionable ringing.
A common solution to correct the ECG for the wideband noise, which may be caused by muscle artifact or electrode movement, has been to pass the ECG signal through a low pass filter having a low cut-off frequency, typically about 25 Hz. Since wideband noise signals typically have significant energy at frequencies higher than 25 Hz, the low pass filter effectively reduces the muscle artifact signal. However, the use of a low pass filter has the disadvantage of also reducing the amplitude of the QRS complex because the high frequency components of the ECG signal which are necessary to describe the relatively high peaks of the QRS signal are also removed as the ECG signal passes through the low pass filter. An example of an approach to filtering muscle artifact in an ECG signal is disclosed in U.S. Pat. No. 5,259,387 granted to depinto and owned by the assignee of the present invention.
Baseline wander is typically caused by interactions between the skin of the patient and the electrode where acids naturally present in the skin of the patient react with metals in the electrode to create an electrolytic reaction on the skin of the patient. This electrolytic reaction, which is similar to the reaction in a battery, provides a relatively constant long term or DC signal which varies slowly over time, producing a low frequency interfering signal superimposed on the electrocardiogram.
One common solution to overcome or minimize the problem of baseline wander is to abrasively remove the top layer of the skin of the patient. Another solution to the problem of baseline wander is to convert the ECG signal to a digital signal on a processing device by creating a cubic spline which passes through the P-R segment of every beat. The cubic spline is then used to determine an offset to be added to the ECG signal to cancel or adjust for the baseline wander. Yet another solution to this problem is to use a high pass filter to remove the slowly varying signals such as baseline wander. Ideally, these filters pass all frequencies above about 0.5 Hz, since baseline wander typically occurs with frequencies below this level and the energy in an ECG signal is primarily above this frequency.
Although the filtering methods described above are effective in reducing or eliminating interfering signals, these filters can also cause a certain amount of distortion of the ECG signal. Even though the magnitude of this distortion may not be sufficient to cause serious problems, it would be preferable to employ the filters only when excessive noise is present and to turn them off at other times. Furthermore, it is preferable to eliminate the source of noise, for example poor electrode preparation or strong electric fields in the vicinity of the patient, instead of turning on a filter.
What is needed is an automatic method of detecting interfering signals and notifying the user. The user may then take corrective action or turn on the appropriate filters. Filters may also be controlled by the automatic noise detection system in such a way as to be turned on when noise is present and turned off when noise levels are low while providing the user with an indication that the filters are on or off and allowing the user to manually override the automatic activation or deactivation of the filters.